Auxiliary switch for an electromagnetic relay

ABSTRACT

One or two auxiliary switches are built into an electromagnetic relay. At each side of the relay, up to three auxiliary terminal inserts are molded in the relay base, each having a screw connector on the outside, and on the inside of the relay housing either a stationary contact, a wire (solder) connector, or both, thus affording a choice of a number of different auxiliary switching and relay coil connection arrangements as auxiliary functions to be performed simultaneously with the main power switching function of the relay. A support for a single-pole double-throw movable contact for the auxiliary switch is molded integrally on each of the left and right ends of the relay contact support that is rigidly secured to the relay armature for rocking movement therewith, and a movable contact is mounted on one or both supports and connected by a wire to the center auxiliary terminal at that side of the base. This provides an auxiliary switch structure having low contact bounce, more overtravel, direct drive, increased reliability and stability under vibration, and economy.

United States Patent Becker et a].

[ AUXILIARY SWITCH FOR AN ELECTROMAGNETIC RELAY [4 1 July 25, 1972 [57] ABSTRACT One or two auxiliary switches are built into an electromagnetic relay. At each side of the relay, up to three auxiliary terminal inserts are molded in the relay base, each having a screw connector on the outside, and on the inside of the relay housing either a stationary contact, a wire (solder) connector, or both, thus affording a choice of a number of different auxiliary switching and relay coil connection arrangements as auxiliary functions to be performed simultaneously with the main power switching function of the relay. A support for a singlepole double-throw movable contact for the auxiliary switch is molded integrally on each of the left and right ends of the relay contact support that is rigidly secured to the relay armature for rocking movement therewith, and a movable contact is mounted on one or both supports and connected by a wire to the center auxiliary terminal at that side of the base. This provides an auxiliary switch structure having low contact bounce, more overtravel, direct drive, increased reliability and stability under vibration, and economy.

8 Claims, 9 Drawing figures [a 12 8 II II 4 25 25 II I 2C i i 2C 34 l 1 40 z c l I lllll a Z8 248 l p, ,8 l 18a j u u q Ill 3+ 245 24 30 AUXILIARY SWITCH FOR AN ELECTROMAGNETIC RELAY BACKGROUND OF THE INVENTION Electromagnetic relays have been provided with auxiliary switches heretofore. However, such prior auxiliary switches have been handicapped by one or more of the disadvantages of excessive contact bounce, insufficient overtravel, indirect coupling from the power contact actuator to the auxiliary contact actuator resulting in unreliable operation, wire connections from the auxiliary switch contacts to the terminals which adds to the cost and might become troublesome and unreliable under shock and vibration, and the like.

While these prior auxiliary switches have been useful for their intended purposes, this invention relates to improvements thereover.

SUMMARY OF THE INVENTION This invention relates to an auxiliary switch and terminal arrangement for an electromagnetic relay.

An object of the invention is to provide an improved auxiliary switch operable by the rockable armature of an electromagnetic relay.

A more specific object of the invention is to provide an improved auxiliary switch wherein the movable contact has initial bias against a stop for low contact bounce and better contact pressure in a small space.

Another specific object of the invention is to provide an improved auxiliary switch that affords more overtravel.

Another specific object of the invention is to provide an improved auxiliary switch for an electromagnetic relay that is directly actuated by the relay contact actuator.

Another specific object of the invention is to provide an improved auxiliary switch that provides increased reliability and stability under vibration and economy by having its stationary contacts connected directly to the terminals.

Other objects and advantages of the invention will hereinafter appear.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a three-pole single-throw electromagnetic relay in which the invention is used;

FIG. 2 is a cross-sectional view taken substantially along line 2-2 of FIG. 3 to show a right side view of the interior of the relay including one auxiliary switch;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2 to show a front view of the interior of the relay including both auxiliary switch contact supports;

FIG. 4 is a cross-section view taken substantially along line 44 OF FIG. 3 to show a left side view of the interior of the relay including the coil connections;

FIG. 5 is an exploded isometric view of the armature and movable contacts subassembly of the relay of FIGS. 1-4 including the auxiliary switch movable contact structure integral therewith;

FIG. 6 is a fragmentary enlarged view of the right auxiliary switch of FIG. 3;

FIG. 7 is a fragmentary enlarged view of a modified auxiliary switch that may be used in the relay of FIGS. 1-5;

FIG. 8 is a fragmentary enlarged view of an alternative use of one of the auxiliary switches of the relay as a normallyclosed signal-pole single-throw switch for coil control purposes; and

FIG. 9 is a schematic diagram showing the circuit in which the auxiliary switch of FIG. 8 may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1-5, there is shown a first application of the invention to a relay of the three-pole single-throw type.

As shown in FIG. 1, the relay comprises an insulating base 2 having a plurality of terminals mounted thereon for connecting the relay to an external circuit. These terminals comprise a first row of three rear terminals 4 connected to respective stationary contacts and a second row of three center terminals 6 connected to respective movable contacts. The latter three temtinals are mounted on a raised portion 2a of the base extending along the center thereof, this raised portion providing space within the relay housing for connector conductors or pigtails" that connect the three movable rocking contacts of the relay to respective terminals 6.

As will be apparent, base 2 has been molded for a singlethrow relay but has space at the front for a row of three more temrinals for connection to stationary contacts to afford use of the same general configuration of base for a double-throw contact relay.

In addition to the above relay terminals, there are provided three auxiliary terminals 8, l0 and 12 at the right side of the base, middle tenninal 10 being on a partially raised portion of the base to provide space therebelow for a wire connection. There may also be provided up to three auxiliary terminals at the left side of the base corresponding to auxiliary terminals 8, l0 and 12, two of these left terminals 14 and 16 being used in the relay illustrated, as shown in FIGS. 1 and 4.

The relay mechanism including its operating magnet and contacts is mounted to the base on the lower side thereof. A metal housing 18 surrounds the relay mechanism and is secured to the base by four screws extending through holes within bosses 2b at the corners of the base. The housing is sealed to the base by an O-ring seal 20 seated in a groove in the base as shown in FIGS. 2 and 3 against which the upper edge of the housing is pressed when the four screws are tightened. A pair of mounting tabs 18a are provided on both the front and back of the housing flush with the bottom. These tabs are provided with holes as shown in FIGS. 1 and 2 for receiving mounting screws, bolts, or the like.

The relay mechanism is put together in three subassemblies, namely, (1) the operating magnet subassembly, (2) the armature and movable contacts subassembly, and (3) the base and stationary contacts subassembly. These three subassemblies are then connected together by connecting the wires to the terminals and mounting the magnet to the base by four screws.

The base and stationary contacts subassembly comprises the aforementioned terminals mounted onto the base as shown in FIGS. 14. Terminals 4 are molded into the insulating base and extend through the base with their upper, external ends being threaded to receive washers and nuts for connection of electrical wires, and their lower internal ends having adjustable stationary contacts formed thereon as shown in J. A. Quaal et al. copending application Ser. No. 155,793, filed June 23, 1971. In the relay shown in FIGS. 1-4, the stationary contacts are arranged to be normally-open with respect to the movable contacts hereinafter described in connection with FIG. 5.

Terminals 6 of this subassembly are also molded into the base at its central raised portion and are provided with threads at their upper,'extemal ends to receive washers and nuts for attachment of electrical conductors in the same manner as terminals 4. The lower, internal ends of these terminals 6 are provided with means for connecting movable contact connectors or pigtails such as ribbon-like connectors 22 shown in FIG. 3. For this purpose, the lower ends of these terminals are provided with tapped bores for receiving screws for securing flexible connectors 22 thereto, these connectors being connected at their other end to the centers of movable contacts 23 shown in FIG. 3 and 5.

The operating magnet and movable contacts subassembly comprises a magnetic frame 24 and an operating coil 26 as shown in FIGS. 2 and 3. This frame comprises a generally E- shaped structure having an armature pivoted on the center branch of this E and being rockable to one or the other pole piece formed on the opposite branches of this E. Frame 24 comprises an L-shaped member 24a of magnetizable material, a round core 24b extending up from member 24a and a permanent magnet PM in spaced parallel relation to core 24b and the vertical part of member 240. This permanent magnet is held in place by a cross-bar 28 overlying it and a pair of screws 30 extending through frame member 24a along opposite sides of this permanent magnet into threaded engagement with the cross-bar. Coil 26 surrounds core 24b, and a plate 32 is secured to the upper end of the core to hold the coil in place and has a pair of studs on its upper surface to provide a pivot for the armature as hereinafter described. Coil terminals 26a are connected by a pair of wires to terminals 14 and 16 as shown in FIGS. 3 and 4.

For securing this operating magnet and movable contacts subassembly -to the base, the base is provided with four downwardly extending, integrally molded studs 2c shown in FIG. 2 having tapped holes in the lower ends thereof for receiving attaching screws. Also, the magnet subassembly is provided with two pairs of laterally extending arms having screw holes therein overlying studs 2c. TWCLOf these arms 24c extend laterally from the vertical part of the frame member 24a as shown in FIGS. 2 and 4. The other two of these arms are the ends of cross-bar 28 as shown in FIG. 3.

The armature and movable contacts subassembly is shown in broken lines in FIGS. 2-4, but is shown more clearly in the exploded view in FIG. 5. As shown therein, armature 34 is a generally rectangular flat piece of magnetizable metal bent slightly at its center so that the two ends are raised and the transverse line at the center forms a pivot for rocking movement of the armature. The armature has a pair of holes 340 spaced apart on the aforesaid transverse center line to receive the aforementioned studs on the upper surface of plate 32 for holding the armature in correct registration on top of the operating magnet and allowing it to rock and actuate the movable contacts carried thereon. Notches 34b may be provided at the opposite ends of the armature for retaining nonmagnetic shims that prevent the armature from sticking to the two poles of the operating magnet. The armature is further provided with four tapped holes 341: for receiving screws that attach the spring support and contact sup ort moldings thereto.

As shown in FIG. 5, the movable contacts are provided with clamping type mounting means for resiliently supporting them on topof the armature. This means comprises a spring support 36 which may be a molding of insulating material having its lower surface formed at an obtuse angle complementary to the obtuse angle on the upper surface of the armature. This spring support rests on top of the armature and is provided with three pairs of shallow depressions 36a, two for each movable contact so that one or two springs may be used for each contact, with the depressions of each pair being on opposite sides of a center ridge 36b. As will be apparent, these pairs of depressions serve to retain the lower ends of one or two movable contact biasing, helical compression springs 38, one spring per contact being shown in FIG. 5, that are placed on top of the spring support. Three movable contacts 23 are then placed on top of the respective springs. Finally, a contact support 40 is placed on top of the movable contacts and held down on ridge 36b to partially depress the springs while four screws 42 are inserted through holes in contact support 40 and spring support 36 and turned into tapped holes 34c in the armature to complete the subassembly.

Spring support 36 is also provided with two partial stepped walls 360 on the side corresponding to the contacting ends of the movable contacts equally spaced so as to form spaces for the movable contacts and to divide the contacting end portions thereof and to lengthen the electrical creepage paths therebetween. These walls also form ridges fitting into underside grooves 40a in contact support 40 to provide interfitting positioning of the contact support on the spring support. In addition, spring support 36 is provided with a rear upwardly extending wall 36d to the level of the slightly higher end portions of the dividing walls 36c to enclose the ends of the movable contacts.

Referring now to contact support 40 in FIG. 5, it will be seen that it supports not only the movable contacts of the relay but also the movable contact of one or two auxiliary switches.

This car tact support 40 is an insulating molding having generally two spaced parallel laterally extending bars 40b that are connected by four transverse bars consisting of a pair of spaced, like middle bars 40c and a pair of like end bars 40d, or: on each of the left and right ends. These four transverse bars are substantially equally spaced from one another to provide three spaces therebetween for the three movable contacts of the three-pole relay. One end portion of each of the two spaced middle bars 400 is provided with the aforementioned underside grooves 40a shown in FIGS. 3 and 5 for receiving dividing walls 360 of the spring support. These spaced middle bars 400 are also each provided with an underside transverse groove 40c shown in FIG. 5 at the center thereof with these grooves being in alignment to receive ridge 36b of the spring support. These interfitting parts provide registration between contact support 40 and spring support 36 and additionally afford three compartments, open at the top, in which the three movable contacts are held.

When the movable q ntacts and springs have been assembled onto spring support 36 and contact support 40 has been placed thereover and screws 42 are tightened, the two lateral bars 40b press down on the movable contacts on opposite sides of center upward offset 230 thereof to provide an initial compressive force on springs 38. Thus, these springs bias the movable contacts upwardly against laterai'bars 40b. In this subassembly, end bars 40d of the contact support provide partial walls for the left and right sides of spring support 36.

The auxiliary switch movable contact supporting means are integrally formed on the relay contact support. This provides one or two single-pole double-throw auxiliary switches or variations thereof that are directly driven-by the relay contact driver.

This means comprises a pair of lateral extensions 40f and 40g on contact support 40 as shown in FIG. 5, onto each of which a generally U-shaped movable contact 44 may be secured by a rivet 46 or the like. Extension 40f has forward and rearward wings 40h and movable contact 44 is formed so that the two arms thereof must be spread apart against the spring force thereof in order for these arms to embrace wings 40h. This provides initial bias of the movable contact arms against their support to reduce contact bounce and to provide better contact pressure in a small space.

Stationary contacts 48 and 50 of the auxiliary switch are connected directly to terminals 8 and 12, respectively, as shown in FIGS. 2 and 6. These terminals 8 and 12 are inserts that are molded into the base and have projections extending down onto which the stationary contacts of the auxiliary switch are riveted. Thus, there are direct electrical connections from the stationary contacts to their terminals without the necessity of any wires or the like with their attendant problems.

Movable contact 44 of the auxiliary switch has a tab 44a shown in FIG. 5 and an dectrical wire 44b connects this tab to center terminal 10 as shown in FIG. 2. As shown in FIG. 3, a vertical hole is provided through the stem of lateral extension 40f through which wire 44b extends to support the wire and prevent vibration thereof.

As shown in FIG. 6, the auxiliary switch has a normallyclosed contact 4448 and a normally-open contact 44-50. When the relay operating magnet is energized, movable contact 44 disengages stationary contact 48 and engages stationary contact 50.

As shown in FIGS. 3 and 4, the left lateral extension 403 is not being used since terminals 14 and 16 are connected to coil terminals 26a.

FIG. 7 shows a modified form of auxiliary switch movable contact support. In this form, the movable contact 52 is substantially U-shaped and is secured within a slot in lateral extension 54f of a contact support 54. This movable contact is formed so that the two arms thereof must be pinched together in order to insert it in the slot and rivet it in. Thus, the outward bias of these arms provides an initial contact bias to reduce bounce and afford enough contact pressure. Lateral extension 54f has inwardly directed wings 54h against which the arms of the movable contact are biased.

A modified current saver" coil connection arrangement is shown in FIGS. 8 and 9. As shown therein, the left lateral extension 40g, instead of being unused as in FIG. 4, is provided with a movable contact 55. Terminal 16 is connected through a connector tab 56 and a wire 58 to one side of the coil, the lower side in FIG. 9. Terminal 14 has a combined stationary contact and connector tab 60 riveted thereto. Thus, terminal 14 is connected through connector tab 60 and a wire 62 to the other side of the coil in FIG. 9. The movable contact is connected by a wire 64 to the center tap of coil 26.

As will be apparent in FIG. 9, terminals 14 and 16 are normally connected through the normally-closed contact 55-60 of the single-pole single-throw auxiliary switch across the lower half of the coil. This causes high current to flow for actuation of the relay. When the relay has been operated, the auxiliary switch contact is open so that for maintaining purposes terminals 14 and 16 will be connected across the entire coil for lower current flow.

At the left side of FIG. 9 is shown schematically the singlepole double-throw auxiliary switch hereinbefore described in connection with FIGS. 2- 5 While the apparatus hereinbefore described is effectively adapted to fulfill the objects stated, it is to be understood that the invention is not intended to be confined to the particular preferred embodiments of auxiliary switch for an electromagnetic relay disclosed, inasmuch as they are susceptible of various modifications without departing from the scope of the appended claims.

We claim:

I. In a relay of the type having an operating mechanism including an operating magnet and an armature pivoted for rocking movement on said operating magnet;

an insulating base;

stationary contact means mounted on the interior side of said base and having terminal means on the exterior side of said base for connection to an external circuit;

movable contact means;

and mounting means for said movable contact means comprising:

and insulating clamping means securing said movable contact means and associated compression spring means to the armature for rocking movement therewith to engage said stationary contact means;

the improvement comprising:

auxiliary switch means operable concurrently with operation of the relay comprising:

auxiliary stationary contact means mounted on the interior side of said base and having terminal means on the exterior side of said base for connection to an external circuit;

a lateral extension on said insulating clamping means operable for movement with said rockable annature;

auxiliary movable contact means mounted on said lateral extension for switching action with said auxiliary stationary contact means;

and terminal means extending through said base and connected to said auxiliary movable contact means.

2. The invention defined in claim 1, wherein:

said auxiliary stationary contact means and associated terminal means comprises inserts molded in said base each having a terminal connector on one end and an auxiliary stationary contact on the other end whereby the auxiliary stationary contacts are connected directly to their respective terminals.

3. The invention defined in claim 2, wherein:

said lateral extension has a tapered, keystone-shaped configuration with short wings;

and said auxiliary movable contact means comprises a generally U-shaped member secured at its bottom to said lateral extension with its arms biased against said wings and the contacting parts of said arms extending above said lateral extension for engagement with said auxiliary stationary contacts.

4. The invention defined in claim 2, wherein:

said lateral extension has a tapered, keystone-shaped slot therein with short projections directed toward one another at the top of said slot;

and said auxiliary movable contact comprises a generally U- shaped strip secured at its bottom to the bottom of said slot and its upwardly directed arms biased outwardly against said short projections with the contacting parts of said arms extending above said lateral extension for engagement with said auxiliary stationary contacts.

5. The invention defined in claim 1, wherein:

said auxiliary stationary contact means and associated terminal means are at one end of said base and similar terminal means are provided at the other end of said base for connection to the coil of the operating magnet.

6. In a relay of the type having an operating mechanism including an operating magnet and an armature pivotally supported for rocking movement on said operating magnet;

an insulating base;

stationary contact means mounted on one side of said base and including terminal means on the other side of said base for connection to an external circuit;

movable contact means for engaging said stationary contact means when said armature is rocked;

and mounting means for said movable contact means comprising:

an insulating spring support on said armature;

compression springs on said spring support;

said movable contact means overlying said compression springs;

an insulating contact support overlying said movable contact means;

and means securing said contact support and said spring support to said armature while affording partial compression of said springs;

the improvement comprising:

auxiliary switch means operable concurrently with operation of the relay comprising:

' auxiliary stationary contacts mounted on said one side of said base on at least one end thereof and including integral terminals on said other side of said base at said one end for connection to an external circuit;

a lateral extension on at least one end of said insulating contact support;

an auxiliary movable contact mounted on said lateral extension for operative engagement with said auxiliary stationary contacts when said armature is rocked;

means on said lateral extension and said auxiliary movable contact affording an initial bias to the latter; and a terminal on said base connected to movable contact.

7. The invention defined in claim 6, wherein:

said auxiliary switch means also comprises a pair of auxiliary terminals at the other end of said base corresponding to said integral terminals and including an auxiliary stationary contact connected to one terminal of said pair;

a like lateral extension on the other end of said insulating contact support;

an auxiliary movable contact mounted on said like lateral extension and having one of its arms normally-closed to said auxiliary stationary contact of said one terminal;

and means connecting the coil of the operating magnet for energizing a portion of the coil through said normallyclosed contact across said pair of auxiliary terminals and for energizing the entire coil across said terminals when said normallyclosed contact opens.

8. The invention defined in claim 6 wherein:

said auxiliary switch means comprises single-pole doublethrow switch contacts.

* t I. i i

said auxiliary @E'llFlCATE 9F QQRREQ'NQN Patent No. 3 680 ,016 Dated August 23, 1972 Inventor(s) Albert H. Becker et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 51, change "section" to --sectional-.

Column 1, line 41, claim 1, insert as additional line "compression spring means for said movable contact means Signed and sealed this 9th day of January 1973..

(SEAL) Attest:'

EDWARD M. FLETCHER,JR.. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,680,016 Dated August 23, 1972 Inventor(s) Albert H. Becker et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 51, change "section" to --sectional--.

Column 1, line 41, claim 1, insert as additional line "compression spring means for said movable contact means;"

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:'

EDWARD M.,FLETCHER,JR., ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. In a relay of the type having an oPerating mechanism including an operating magnet and an armature pivoted for rocking movement on said operating magnet; an insulating base; stationary contact means mounted on the interior side of said base and having terminal means on the exterior side of said base for connection to an external circuit; movable contact means; and mounting means for said movable contact means comprising: and insulating clamping means securing said movable contact means and associated compression spring means to the armature for rocking movement therewith to engage said stationary contact means; the improvement comprising: auxiliary switch means operable concurrently with operation of the relay comprising: auxiliary stationary contact means mounted on the interior side of said base and having terminal means on the exterior side of said base for connection to an external circuit; a lateral extension on said insulating clamping means operable for movement with said rockable armature; auxiliary movable contact means mounted on said lateral extension for switching action with said auxiliary stationary contact means; and terminal means extending through said base and connected to said auxiliary movable contact means.
 2. The invention defined in claim 1, wherein: said auxiliary stationary contact means and associated terminal means comprises inserts molded in said base each having a terminal connector on one end and an auxiliary stationary contact on the other end whereby the auxiliary stationary contacts are connected directly to their respective terminals.
 3. The invention defined in claim 2, wherein: said lateral extension has a tapered, keystone-shaped configuration with short wings; and said auxiliary movable contact means comprises a generally U-shaped member secured at its bottom to said lateral extension with its arms biased against said wings and the contacting parts of said arms extending above said lateral extension for engagement with said auxiliary stationary contacts.
 4. The invention defined in claim 2, wherein: said lateral extension has a tapered, keystone-shaped slot therein with short projections directed toward one another at the top of said slot; and said auxiliary movable contact comprises a generally U-shaped strip secured at its bottom to the bottom of said slot and its upwardly directed arms biased outwardly against said short projections with the contacting parts of said arms extending above said lateral extension for engagement with said auxiliary stationary contacts.
 5. The invention defined in claim 1, wherein: said auxiliary stationary contact means and associated terminal means are at one end of said base and similar terminal means are provided at the other end of said base for connection to the coil of the operating magnet.
 6. In a relay of the type having an operating mechanism including an operating magnet and an armature pivotally supported for rocking movement on said operating magnet; an insulating base; stationary contact means mounted on one side of said base and including terminal means on the other side of said base for connection to an external circuit; movable contact means for engaging said stationary contact means when said armature is rocked; and mounting means for said movable contact means comprising: an insulating spring support on said armature; compression springs on said spring support; said movable contact means overlying said compression springs; an insulating contact support overlying said movable contact means; and means securing said contact support and said spring support to said armature while affording partial compression of said springs; the improvement comprising: auxiliary switch means operable concurrently with operation of the relay comprising: auxiliary stationary contacts mounted on said one side of said base on at least one end thereof and including integral terminals on said other side of said base at said one end for connection to an external circuit; a lateral extension on at least one end of said insulating contact support; an auxiliary movable contact mounted on said lateral extension for operative engagement with said auxiliary stationary contacts when said armature is rocked; means on said lateral extension and said auxiliary movable contact affording an initial bias to the latter; and a terminal on said base connected to said auxiliary movable contact.
 7. The invention defined in claim 6, wherein: said auxiliary switch means also comprises a pair of auxiliary terminals at the other end of said base corresponding to said integral terminals and including an auxiliary stationary contact connected to one terminal of said pair; a like lateral extension on the other end of said insulating contact support; an auxiliary movable contact mounted on said like lateral extension and having one of its arms normally-closed to said auxiliary stationary contact of said one terminal; and means connecting the coil of the operating magnet for energizing a portion of the coil through said normally-closed contact across said pair of auxiliary terminals and for energizing the entire coil across said terminals when said normally-closed contact opens.
 8. The invention defined in claim 6 wherein: said auxiliary switch means comprises single-pole double-throw switch contacts. 